The present invention relates to the fabrication of circuit boards for supporting and interconnecting electronic components, and more particularly, to a method of mass producing metallized injection molded plastic substrates that results in substantial savings in tooling and handling costs.
Conventional printed circuit boards are fabricated from sheets of copper clad FR-4 epoxy glass bonded laminate substrate material. The substrate material is drilled to provide component lead holes, and then scrubbed and deburred. A thin layer of copper is deposited on the substrate through electroless plating in order to metallize the holes. Thereafter photoresist masking and imaging processes are used to delineate the desired conductive pattern. A relatively thick layer of copper or other metal is deposited through electro-plating and then chemical etching is done to remove the excess copper, leaving the desired conductive patterns on the substrate.
The aforementioned conventional process for fabricating printed circuit boards is labor intensive, requires substantial expenditures for heavy equipment, has time consuming multiple steps, and in addition produces effluents which must be specially handled to reduce environmental hazards. Significant percentages of defective circuit boards can result unless close registration tolerances are maintained during the drilling, masking and imaging steps.
In my U.S. Pat. No. 4,532,152 entitled "Fabrication of a Printed Circuit Board with Metal-Filled Channels" there are disclosed various processes of fabricating a printed circuit board by injection molding a plastic substrate to provide holes and channels therein. The channels are metallized to delineate a recessed conductive pattern. These processes have significant advantages in that the drilling, routing, deburring, masking and imaging steps are eliminated. The resulting printed circuit boards are thus cheaper to produce and have less defects that conventional FR-4 laminate printed circuit boards.
Conventional printed circuit boards are often photolithographically delineated in large arrays on a single large sheet of copper clad FR-4 laminate substrate. Sections of the large single sheet, each containing an individual circuit, are then "pierced and replaced", i.e. a cutting die is used to stamp out the sections which are then re-inserted snugly into the resulting receptacles. These arrays are then shpped to customers. A plurality of the circuit boards are then simultaneously plugged or "stuffed" with electronic components by the customer using an automatic insertion machine and then wave soldered. The individual boards are then pressed out of their receptacles by the customer.
Printed circuit boards must conform to an industry standard, which usually requires a one-sixteenth inch thickness. When injection molded printed circuit boards are produced, it is important that the boards have a controlled uniform thickness and that the channels and holes are accurately formed. Molds can be made that will delineate an arraya of individual circuit boards on a common substrate so that the individual boards may be simultaneously pierced and replaced, metallized, plugged, stuffed, wave soldered and then pressed out. However, if the surface area of the array becomes too large, the cost of the tooling becomes prohibitive, the pressure required in the injection molding machine becomes very large, and it becomes increasingly difficult to maintain the necessary tolerances. By way of example, at the present time it is economically feasible to produce molds that can be used to simultaneously form eight identical circuit boards measuring approximately three and one-quarter inches by one and three-quarter inches on a single common substrate measuring eight inches square. The aforementioned molding can be achieved on a three hundred ton press with adequate tolerance with regard to the uniformity of thickness and accuracy of the location of the channels and holes. By way of comparison, the cost of molds to simultaneously form forty-eight of the same size circuit boards on a common substrate measuring sixteen inches by twenty-four inches might be six to seven times as much, the mold pressure required might be six times as much and significant problems in holding to adequate tolereances would be encountered.
It would be desirable to provide a method of manufacturing molded printed circuit boards that would take advantage of the savings in handling that result from large array or batch processing without incurring the significant expense and tolerance problems associated with using large injection molds to form a common plastic substrate having the substantial area required for many individual boards.